Synthetic resin materials, especially polymers of readily polymerizable, olefinically unsaturated compounds; in addition to material such as amalgam, gold and porcelain, are used for making tooth fillings, tooth sealings and facings for crowns, bridges and for replacing teeth. Synthetic resins offer the advantage of being acceptable from a cosmetic point of view relative to the other materials and are impact resistant, unlike porcelain, for example.
About 45 years ago poly(methyl methacrylate) was used for the first time for the above-identified purposes. However, the preparation of tooth fillings required that the polymerization can be carried out only at normal or body temperature. As a result, however, a small portion of the methyl methacrylate invariably remains unpolymerized in most cases causing death of the pulp because it has been proven that residual monomers gradually diffuse out of the filling resulting in such damage.
During the past few decades the dental industry has substituted these polymers with polyfunctional methacrylic acid esters such as bisglycidyl methacrylates of isopropylidene phenol which are described in U.S. Pat. No. 3,066,122 and are known by the name of bis-GMA. Several disadvantages of these monomers including high viscosity and swelling in water could be overcome by a modification thereof with isocyanates (U.S. Pat. No. 3,629,187) or by the use of other diphenols or alkanediols. Examples for such monomers are given in German Patent Nos. 1,921,869, 2,816,823 and 2,934,380 or U.S. Pat. Nos. 3,629,187 and 4,406,625.
In addition, German Inspection Specification 25 41 641 discloses that radiation-hardening binders on the basis of unsaturated polyesters are known and are to be used preferably in solvent-free printing inks.
However, all the hitherto described monomers are subject to substantial shrinkage upon polymerization so that large quantities of inorganic filler must be added in order to control the shrinkage within acceptable limits. Nevertheless, the existing polymerization shrinkage still regularly results in a marginal gap between the tooth and the synthetic resin filling, or between the metal and the synthetic resin facing which results in poor adhesion and which allows bacteria to enter the gap thereby initiating secondary caries.
The use of the etch-and-bond technique or the use of so-called dentine adhesives allows the polymerization shrinkage to be displaced from the margin of the filling into the filling itself, so that despite a certain polymerization shrinkage the adhesion between the tooth and the synthetic resin filling or between the metal and the synthetic resin facing is warranted. However, this requires a highly sophisticated processing technique. Thus, processing defects can be avoided only by utmost care and become apparent only after 1 to 2 years much to the patient's regret.